Earlier quoted context omitted.
That's the exact purpose of stock options. From the IRS's perspective, when you receive shares in a private company, that's still income, because those shares have a specific value (determined by 409a valuation), despite them being illiquid.
Why the need for an exercise price(you can always keep it at a nominal amount like 1 cent) and short windows of redemption once you leave then? From most terms of options I have seen or heard of they have been quite restrictive compared to how flexible RSUs are.
If you grant stock options with an exercise price that is less than the fair market value (share price from 409a), then that's considered income by the IRS and employees owe tax immediately.
With ISOs, the 90 day exercise window is required by the IRS. With NSOs, it can be longer, like 7-10 years. 7-10 years is obviously way more employee friendly.